1. Field of the Invention
The present invention relates to an elastic wave filter device.
2. Description of the Related Art
Elastic wave filter devices utilizing elastic waves, such as surface acoustic waves or boundary acoustic waves, have been widely used as filter devices. As one type of such elastic wave filter devices, a ladder surface acoustic wave filter device is disclosed in Japanese Unexamined Patent Application Publication No. 2004-72411. FIG. 14 illustrates a schematic circuit diagram of the surface acoustic wave filter device described in Japanese Unexamined Patent Application Publication No. 2004-72411.
In a surface acoustic wave filter device 100 illustrated in FIG. 14, multiple series arm resonators S101 to S103 are connected in series in a series arm 103 that connects signal terminals 101 and 102. First and second parallel arms 104 and 105 are provided between the series arm 103 and a ground potential. The parallel arms 104 and 105 are provided with parallel arm resonators P101 and P102, respectively. An inductor L101 is connected between the parallel arm resonator P101 and the ground potential. An inductor L102 is connected between the parallel arm resonator P102 and the ground potential. An inductor L103 is connected between a connection point of the inductors L101 and L102 and the ground potential. A capacitor C101 is connected between a connection point of the parallel arm resonator P101 and the inductor L101 and a connection point of the parallel arm resonator P102 and the inductor L102.
Japanese Unexamined Patent Application Publication No. 2004-72411 describes that attenuation at frequencies lower than those of a pass band can be increased by providing the inductors L101 to L103 and the capacitor C101.
In elastic wave filter devices, there has been a demand for an increase in attenuation of harmonics, e.g., second and third harmonics of a pass band. However, the surface acoustic wave filter device 100 is incapable of sufficiently increasing attenuation of harmonics.